The control of various types of electrical loads by operating solid state switch means in response to a condition sensing means is well known. Typically a solid state switch means, such as a triac, is controlled in response to a voltage variation at its gate determined by the magnitude of a slowly varying resistance in a condition sensing circuit. The slowly varying resistance would typically be an element such as a thermistor or a cadmium sulfide cell. In order to create as definite a switching function as possible, the gate of the triac is connected to the condition responsive element by a voltage breakdown type of device or silicon bilateral switch.
Even though in theory this provides a definite switching point for the triac, actually two possible switching conditions are created because the bilateral switches are not perfectly symmetrical. That is, the breakdown potential from positive to negative, and from negative to positive, may be slightly different. In certain cases where the system is operated with an alternating current potential in the sensing or gate circuit, the triac can be caused to be gated on one-half cycle of the applied alternating current voltage while remaining nonconductive on the reverse half cycle of the applied alternating current potential. This is referred to as "half-waving".
If the condition being sensed changes quite rapidly, this problem is relatively unimportant. If a very slowly changing condition exists, that is one in which the impedance of the sensing element is changing relatively slowly to the applied alternating current potential, the two halves of the gating signal to the triac can be sufficiently different so that the triac will operate on one-half cycle of the applied alternating current and will not conduct on the reverse half cycle. In certain types of applications of cadmium sulfide sensors to fuel burners, this type of operation can be very undesirable.